package com.hm.common.util;

import java.util.NoSuchElementException;
import java.util.Objects;
import java.util.function.Consumer;
import java.util.function.Function;
import java.util.function.Predicate;
import java.util.function.Supplier;

/**
 * @author shishun.wang
 * @date 2018年1月18日 上午10:58:35
 * @version 1.0
 * @describe
 */
public final class Optional<T> {
	/**
	 * Common instance for {@code empty()}.
	 */
	private static final Optional<?> EMPTY = new Optional<>();

	/**
	 * If non-null, the value; if null, indicates no value is present
	 */
	private final T value;

	/**
	 * Constructs an empty instance.
	 *
	 * @implNote Generally only one empty instance, {@link Optional#EMPTY}, should
	 *           exist per VM.
	 */
	private Optional() {
		this.value = null;
	}

	/**
	 * Returns an empty {@code Optional} instance. No value is present for this
	 * Optional.
	 *
	 * @apiNote Though it may be tempting to do so, avoid testing if an object is
	 *          empty by comparing with {@code ==} against instances returned by
	 *          {@code Option.empty()}. There is no guarantee that it is a
	 *          singleton. Instead, use {@link #isPresent()}.
	 *
	 * @param <T>
	 *            Type of the non-existent value
	 * @return an empty {@code Optional}
	 */
	public static <T> Optional<T> empty() {
		@SuppressWarnings("unchecked")
		Optional<T> t = (Optional<T>) EMPTY;
		return t;
	}

	/**
	 * Constructs an instance with the value present.
	 *
	 * @param value
	 *            the non-null value to be present
	 * @throws NullPointerException
	 *             if value is null
	 */
	private Optional(T value) {
		this.value = Objects.requireNonNull(value);
	}

	/**
	 * Returns an {@code Optional} with the specified present non-null value.
	 *
	 * @param <T>
	 *            the class of the value
	 * @param value
	 *            the value to be present, which must be non-null
	 * @return an {@code Optional} with the value present
	 * @throws NullPointerException
	 *             if value is null
	 */
	public static <T> Optional<T> of(T value) {
		if (null == value) {
			return empty();
		}
		return new Optional<>(value);
	}

	/**
	 * Returns an {@code Optional} describing the specified value, if non-null,
	 * otherwise returns an empty {@code Optional}.
	 *
	 * @param <T>
	 *            the class of the value
	 * @param value
	 *            the possibly-null value to describe
	 * @return an {@code Optional} with a present value if the specified value is
	 *         non-null, otherwise an empty {@code Optional}
	 */
	public static <T> Optional<T> ofNullable(T value) {
		return value == null ? empty() : of(value);
	}

	/**
	 * If a value is present in this {@code Optional}, returns the value, otherwise
	 * throws {@code NoSuchElementException}.
	 *
	 * @return the non-null value held by this {@code Optional}
	 * @throws NoSuchElementException
	 *             if there is no value present
	 *
	 * @see Optional#isPresent()
	 */
	public T get() {
		if (value == null) {
			throw new NoSuchElementException("No value present");
		}
		return value;
	}

	/**
	 * Return {@code true} if there is a value present, otherwise {@code false}.
	 *
	 * @return {@code true} if there is a value present, otherwise {@code false}
	 */
	public boolean isPresent() {
		return value != null;
	}

	/**
	 * If a value is present, invoke the specified consumer with the value,
	 * otherwise do nothing.
	 *
	 * @param consumer
	 *            block to be executed if a value is present
	 * @throws NullPointerException
	 *             if value is present and {@code consumer} is null
	 */
	public void ifPresent(Consumer<? super T> consumer) {
		if (value != null)
			consumer.accept(value);
	}

	/**
	 * If a value is present, and the value matches the given predicate, return an
	 * {@code Optional} describing the value, otherwise return an empty
	 * {@code Optional}.
	 *
	 * @param predicate
	 *            a predicate to apply to the value, if present
	 * @return an {@code Optional} describing the value of this {@code Optional} if
	 *         a value is present and the value matches the given predicate,
	 *         otherwise an empty {@code Optional}
	 * @throws NullPointerException
	 *             if the predicate is null
	 */
	public Optional<T> filter(Predicate<? super T> predicate) {
		Objects.requireNonNull(predicate);
		if (!isPresent())
			return this;
		else
			return predicate.test(value) ? this : empty();
	}

	/**
	 * If a value is present, apply the provided mapping function to it, and if the
	 * result is non-null, return an {@code Optional} describing the result.
	 * Otherwise return an empty {@code Optional}.
	 *
	 * @apiNote This method supports post-processing on optional values, without the
	 *          need to explicitly check for a return status. For example, the
	 *          following code traverses a stream of file names, selects one that
	 *          has not yet been processed, and then opens that file, returning an
	 *          {@code Optional<FileInputStream>}:
	 *
	 *          <pre>
	 * {@code
	 *     Optional<FileInputStream> fis =
	 *         names.stream().filter(name -> !isProcessedYet(name))
	 *                       .findFirst()
	 *                       .map(name -> new FileInputStream(name));
	 * }
	 *          </pre>
	 *
	 *          Here, {@code findFirst} returns an {@code Optional<String>}, and
	 *          then {@code map} returns an {@code Optional<FileInputStream>} for
	 *          the desired file if one exists.
	 *
	 * @param <U>
	 *            The type of the result of the mapping function
	 * @param mapper
	 *            a mapping function to apply to the value, if present
	 * @return an {@code Optional} describing the result of applying a mapping
	 *         function to the value of this {@code Optional}, if a value is
	 *         present, otherwise an empty {@code Optional}
	 * @throws NullPointerException
	 *             if the mapping function is null
	 */
	public <U> Optional<U> map(Function<? super T, ? extends U> mapper) {
		Objects.requireNonNull(mapper);
		if (!isPresent())
			return empty();
		else {
			return Optional.ofNullable(mapper.apply(value));
		}
	}

	/**
	 * If a value is present, apply the provided {@code Optional}-bearing mapping
	 * function to it, return that result, otherwise return an empty
	 * {@code Optional}. This method is similar to {@link #map(Function)}, but the
	 * provided mapper is one whose result is already an {@code Optional}, and if
	 * invoked, {@code flatMap} does not wrap it with an additional
	 * {@code Optional}.
	 *
	 * @param <U>
	 *            The type parameter to the {@code Optional} returned by
	 * @param mapper
	 *            a mapping function to apply to the value, if present the mapping
	 *            function
	 * @return the result of applying an {@code Optional}-bearing mapping function
	 *         to the value of this {@code Optional}, if a value is present,
	 *         otherwise an empty {@code Optional}
	 * @throws NullPointerException
	 *             if the mapping function is null or returns a null result
	 */
	public <U> Optional<U> flatMap(Function<? super T, Optional<U>> mapper) {
		Objects.requireNonNull(mapper);
		if (!isPresent())
			return empty();
		else {
			return Objects.requireNonNull(mapper.apply(value));
		}
	}

	/**
	 * Return the value if present, otherwise return {@code other}.
	 *
	 * @param other
	 *            the value to be returned if there is no value present, may be null
	 * @return the value, if present, otherwise {@code other}
	 */
	public T orElse(T other) {
		return value != null ? value : other;
	}

	/**
	 * Return the value if present, otherwise invoke {@code other} and return the
	 * result of that invocation.
	 *
	 * @param other
	 *            a {@code Supplier} whose result is returned if no value is present
	 * @return the value if present otherwise the result of {@code other.get()}
	 * @throws NullPointerException
	 *             if value is not present and {@code other} is null
	 */
	public T orElseGet(Supplier<? extends T> other) {
		return value != null ? value : other.get();
	}

	/**
	 * Return the contained value, if present, otherwise throw an exception to be
	 * created by the provided supplier.
	 *
	 * @apiNote A method reference to the exception constructor with an empty
	 *          argument list can be used as the supplier. For example,
	 *          {@code IllegalStateException::new}
	 *
	 * @param <X>
	 *            Type of the exception to be thrown
	 * @param exceptionSupplier
	 *            The supplier which will return the exception to be thrown
	 * @return the present value
	 * @throws X
	 *             if there is no value present
	 * @throws NullPointerException
	 *             if no value is present and {@code exceptionSupplier} is null
	 */
	public <X extends Throwable> T orElseThrow(Supplier<? extends X> exceptionSupplier) throws X {
		if (value != null) {
			return value;
		} else {
			throw exceptionSupplier.get();
		}
	}

	/**
	 * Indicates whether some other object is "equal to" this Optional. The other
	 * object is considered equal if:
	 * <ul>
	 * <li>it is also an {@code Optional} and;
	 * <li>both instances have no value present or;
	 * <li>the present values are "equal to" each other via {@code equals()}.
	 * </ul>
	 *
	 * @param obj
	 *            an object to be tested for equality
	 * @return {code true} if the other object is "equal to" this object otherwise
	 *         {@code false}
	 */
	@Override
	public boolean equals(Object obj) {
		if (this == obj) {
			return true;
		}

		if (!(obj instanceof Optional)) {
			return false;
		}

		Optional<?> other = (Optional<?>) obj;
		return Objects.equals(value, other.value);
	}

	/**
	 * Returns the hash code value of the present value, if any, or 0 (zero) if no
	 * value is present.
	 *
	 * @return hash code value of the present value or 0 if no value is present
	 */
	@Override
	public int hashCode() {
		return Objects.hashCode(value);
	}

	/**
	 * Returns a non-empty string representation of this Optional suitable for
	 * debugging. The exact presentation format is unspecified and may vary between
	 * implementations and versions.
	 *
	 * @implSpec If a value is present the result must include its string
	 *           representation in the result. Empty and present Optionals must be
	 *           unambiguously differentiable.
	 *
	 * @return the string representation of this instance
	 */
	@Override
	public String toString() {
		return value != null ? String.format("Optional[%s]", value) : "Optional.empty";
	}
}
